Captain McCannon looked furtively around the room. "Bugger classes," she muttered under her breath. "I'd be better off doing my own work." Turning her back on her programming teacher, she quickly opened up the Brass Goggles wiki, fired up Pandora, and was soon off on a ska-fueled editing tangent the likes of which the wiki had never seen.
The above was a rather silly example of roleplaying (that is, if I were pretending to be an easily distracted robotics student, as opposed to actually being one). The main principle of roleplaying, or RP, or - in Brass Goggles' case - portrayal, is to write part of a story from the point-of-view of a character you make up.
The Basic Mechanics
Here's how roleplaying on Brass Goggles (and other forums, to an extent) works:
Usually, one writes up a profile of their character. A lot of people have a pet character or persona they like using a lot, but also have alternate characters they play.
Once this is accomplished, they jump into a Portrayal thread. This is usually written from an omniscient third person point-of-view.
Alternately, one can start a new Portrayal thread. It is advised, however, that one lurks around the board and posts in another thread before doing this. New threads are like new stories, with a new setting and a plot which the characters can finish.